Computation of soft bits for a turbo decoder in a communication receiver

ABSTRACT

A device ( 20 ) for computing a threshold Sthi used in demodulating a quadrature amplitude modulated (QAM) signal to generate a plurality of soft bits per received symbol for input to a turbo decoder, the device including: means ( 30 ) for computing the mean amplitude A of the received symbols and multiplying the mean amplitude A by of the received symbols and multiplying the mean amplitude A by a constant C i  for a square QAM constellation with 4 m  points, such that Sth i =A×C i  where m is a positive integer and i is a positive integer from 1 to (√4 m-1 )−1.

TECHNICAL FIELD

The present invention relates generally to demodulation of a quadratureamplitude modulation signal to generate soft bits for input to a channeldecoder in a communication receiver and in particular in a spreadspectrum communication receiver.

BACKGROUND ART

Communication systems employ forward error correction in order tocorrect errors caused by noise generated in transport channels. Forexample, a communication system typically uses a turbo code for theforward error correction. At the transmitter side, turbo encoderintroduces redundancy bits based on information bits. The encoded bitsat the output of turbo encoder are then modulated and transmitted to thereceiver. At the receiver end, the receiver demodulates the receivedsignal and produce received encoded bits to the turbo decoder. Turbodecoder decodes the received encoded bits to recover the informationbits.

To maximize the advantage of the coding gain obtained by the iterativedecoding process in the turbo decoder, rather than determiningimmediately whether received encoded bits are zero or one, thecommunication receiver assigns each bit of value on a multi level scalerepresentative of the probability that the bit is 1. A common scale,referred to as Log-Likelihood Ratio (LLR) probabilities, represents eachbit in an integer in some range, for example −32 to 31. The value of 31signifies that the transmitted bit was 0 with a very high probability,and a value of −32 signifies that the transmitted bit was 1 with a veryhigh probability. A value of 0 indicates that the logical bit value isindeterminate.

The turbo decoder in a communication receiver uses the LLR to determinewhether a given information bit were transmitted given a particularreceived encoded bits. However, the computation of the LLR probabilitiesis a time-consuming and processing-intensive calculation. By way ofexplanation, in a transmitter of a communication system, each N encodedbits are mapped to one symbol (2 dimensional symbol with I and Qcomponents). The symbol is transmitted over a channel to reach areceiver. The received symbol is attenuated and corrupted by noise. Thetask of the receiver's demodulator is to recover the N encoded bits fromthat noisy received symbol. To utilize the coding gain of the turbodecoder, N soft bits (or LLR) are generated.

Usually, the log likelihood is used to approximate the soft bit. The loglikelihood L(b_(i)) for i-th bit (i=0, 1, . . . , N−1) is calculated as:

$\begin{matrix}{{L\left( b_{i} \right)} = {\ln\frac{P\left( {b_{i} = {0❘y}} \right)}{P\left( {b_{i} = {1❘y}} \right)}}} \\{= {\ln\frac{\sum\limits_{{z❘b_{i}} = 0}{P\left( {z❘y} \right)}}{\sum\limits_{{z❘b_{i}} = 1}{P\left( {z❘y} \right)}}}} \\{\approx {\frac{1}{2\sigma^{2}}\left( {{\min_{{z❘b_{i}} = 1}{{y - z}}^{2}} - {\min_{{z❘b_{i}} = 0}{{y - z}}^{2}}} \right)}}\end{matrix}$where y is the received symbol, z is a symbol in the referenceconstellation, and σ² is noise variance. From this formula, it can beseen that significant computational complexity is involved in theestimation of σ², the estimation of the reference constellation(estimation of average amplitude of the desired signal), the calculationof the distances and min searches and the division to derive L(b_(i))

In order to simplify the log likelihood computation, a simplified methodis proposed in International Patent Application No WO 01/67617 forgenerating the soft decisions. A particular example described in thatdocument is the demodulation of a 16 Quadrature Amplitude Modulation(QAM) signal in a spread spectrum communication system. A 16 QAMconstellation is shown in FIG. 1, where each of the 16 symbols in theconstellation corresponds to 4 encoded bits. FIG. 2 highlights therelationship between the encoded bits and the location of each symbol.For example, if b0=1 and b1=0, the symbol has negative I and positive Qcomponent. Based on similar observations for b1 and b3, soft bitsL(b_(i)) can be generated as follows:L(b ₀)=yI,L(b ₁)=yQ,L(b ₂)=Sth−abs(yI),L(b ₃)=Sth−abs(yQ)However, a complex circuit must be used in the arrangement described inthe above-mentioned document for calculation of Sth_(i), which involves:

(i) calculation of interference power and calculation of desired signalpower for each multipath based on pilot signal, and

(ii) mapping signal and interference power to C/I for each path, andsumming to get total C/I, which is then used as an estimate of thethreshold Sth_(i)

Accordingly, there currently remains a need to compute log-likelihoodratios for use by a turbo decoder in a computationally simple manner.

DISCLOSURE OF THE INVENTION

It would be desirable to provide a technique for computing a thresholdvalue used in the computation of log-likelihood metrics that minimisesthe time, processing resources and power requirements of a communicationreceiver.

It would also be desirable to provide a technique for calculatinglog-likelihood ratio metrics in a manner that ameliorates or overcomesone or more disadvantages of known techniques.

With this in mind, one aspect of the invention provides a method forcomputing a threshold Sthi used in demodulating a quadrature amplitudemodulated (QAM) signal to generate a plurality of soft bits per receivedsymbol for input to a turbo decoder, the method including the steps of:

computing the mean amplitude A of the received symbols; and

multiplying the mean amplitude A by a constant C_(i) for a square QAMconstellation with 4^(m) points, such thatSth _(i) =A×C _(i)where m is a positive integer and i is a positive integer from 1 to(√{square root over (4^(m-1))})−1.

A method including these steps efficiently calculates the threshold usedin demodulating a QAM signal to generate a plurality of soft bits perreceived symbol for input to a turbo decoder in High Speed DownlinkPacket Access (HSDPA) applications. The threshold is computed as aproduct of the average amplitude of the received signal and a constantor fixed scaling factor. The estimation of average amplitude of thereceived signal is relatively straight forward to computed and can beperformed efficiently in hardware.

In one embodiment, the mean amplitude A is computed from a block of Kreceived symbols, where K is a positive integer.

The value of K may be inversely proportional to the speed of change inchannel conditions.

The constant C_(i) may be computed according toC _(i)=2×I×Δwhere Δ is a normalising parameter for a square QAM constellation with4^(m) points.

In one embodiment, the QAM signal is a 16 QAM signal and the constantC_(i) equals

$\frac{2}{\sqrt{10}}.$

In another embodiment, the QAM signal is a 16 QAM signal and theconstant C_(i) equals 0.5.

The mean amplitude A of the received symbols may be computed accordingtoA=max(AI,AQ)+0.5min(A1, AQ)where AI and AQ are respectively the averages of orthogonal I and Qcomponents of each received symbol.

Alternatively, the mean amplitude A of the received symbols may becomputed according toA=AI+AQwhere AI and AQ are respectively the averages of orthogonal I and Qcomponents of each received symbol.

Another aspect of the invention provides a method for generating softbits per received symbol for input to a turbo decoder used indemodulating a quadrature amplitude modulated (QAM) signal, the methodincluding the steps of:

computing the threshold Sth_(i) as described above; and

computing one or more of the soft bits from the threshold Sth_(i).

As has been explained previously, an efficient way of demodulating a QAMsignal is to generate a plurality of soft bits for input to a turbodecoder usually involves calculation of a threshold. A demodulatortypically produces the first two soft bits equal to an I component and Qcomponent of a received symbol. One or more of the remaining soft bitsare able to be generated according to the present invention asdifferences between the absolute values of the I component and Qcomponent and the threshold. The simple manner in which the threshold isderived therefore minimises the complexity of the required hardware in acommunication receiver.

In one embodiment of the invention, log₂ 4^(m) soft bits are computedfrom the threshold Sth_(i).

Yet another aspect of the invention provides a device for computing athreshold Sth_(i) used in demodulating a quadrature amplitude modulated(QAM) signal to generate a plurality of soft bits per received symbolfor input to a turbo decoder, the device including:

means for computing the mean amplitude A of the received symbols andmultiplying the mean amplitude A by a normalising parameter Δ for asquare QAM constellation with 4^(m) points, such thatSth _(i) =A×C _(i)where m is a positive integer and i is a positive integer from 1 to(√{square root over (4^(m-1))})−1.

A further aspect of the invention provides a device for generating softbits per received symbol for input to a turbo decoder used indemodulating a quadrature amplitude modulated (QAM) signal, the deviceincluding:

means for computing the mean amplitude A of the received symbols andmultiplying the mean amplitude A by a constant C_(i) for a square QAMconstellation with 4^(m) points, such thatSth _(i) =A×C _(i)where m is a positive integer and i is a positive integer from 1 to(√{square root over (4^(m-1))})−1; and

means for computing one or more of the soft bits from the thresholdSth_(i).

A still further aspect of the invention provides a communicationreceiver including a device as described above.

The various features and advantages of the present invention will becomemore apparent from the following detailed description when taken inconjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagram of a 16 QAM signal constellation mapping;

FIG. 2 is a diagram of the constellation mapping showing the value ofeach of 4 encoded bits within a grey code representing each symbol inthe constellation mapping;

FIG. 3 is a schematic diagram of a communication system including acommunication receiver in accordance with one embodiment of the presentinvention;

FIG. 4 is a schematic diagram of a first embodiment of a soft bitestimator forming part of the communication receiver shown in FIG. 3;and

FIG. 5 is a schematic diagram of a second embodiment of a soft bitestimator forming part of the communication receiver shown in FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 3, there is shown generally a communication system10 including a transmitter 11 connected to a digital source 12 andconfigured to receive binary input data from the digital source 12. Thetransmitter 11 generates and transmits a modulated signal 13 forreception by a receiver 14. The receiver 14 acts to demodulate thereceived signal and recover decoded binary data which is then forwardedto a destination 15. The transmitter 11 includes a turbo encoder 16,signal mapping block 17 and modulator 18. The receiver includes ademodulator 19, log-likelihood estimator 20 and turbo decoder 21.

Binary input data 22 to be transmitted is encoded with a turbo code bythe turbo encoder 16, which generates a sequence of binary symbols 23referred to as encoded bits. Several encoded bits are blocked togetherand mapped to a point on a signal constellation by the signal mappingblock 17, thereby generating a sequence of complex-valued modulationsymbols 24. This sequence is applied to the modulator 18, whichgenerates a continuous-time wave form for transmission to the receiver14.

The demodulator 19 demodulates the received modulated signal andgenerates a sequence of complex-valued soft symbol 25. Each soft symbolrepresents an estimate of a modulation symbol that was transmitted bythe transmitter 11. These estimates are used by the log-likelihoodestimator 20 to extract log-likelihood metrics (soft bits) 26 associatedwith the given modulation symbol. The turbo decoder 21 uses the sequenceof log-likelihood metrics (soft bits) to decode the binary data that wasoriginally transmitted and recover decoded binary data 27.

The square 16 QAM constellation shown in FIG. 1 has an index m=2 and isdefined to be a signal constellation with 4^(m) points. Each signalpoint is denoted by its index (i,j) where 0≦i, j<2^(m). The position ofeach i,j point on the constellation is given by the following formula:

$\sqrt{\frac{3}{4^{m} - 1}\left( \frac{1}{2} \right)}.$

The above formula ensures that the average energy of the signalconstellation is normalised to one, where Δ is a normalisation parameterfor a square QAM constellation. For a 16 QAM constellation, m=2 and Δ=

$\frac{1}{\sqrt{10}}.$For other square QAM constellations, the value of both m and Δ willchange. Accordingly, for a 64 QAM constellation, m=3 and Δ=

$\frac{1}{\sqrt{42}},$whilst for a 256 QAM constellation, m=4 and Δ=

$\frac{1}{\sqrt{170}}.$

Each signal pointing the constellation is labelled with a binary stringthat denotes a block of encoded bits associated with the modulationsymbol. As can be seen in FIG. 1, grey code mapping is used to associatethe modulation symbols with blocks of encoded bits. In this case, eachof the 4^(m) points in the constellation is identified by a 4 bit codeincluding bits b0, b1, b2 and b3. In other square QAM constellations,the number of bits in the grey code may vary. For example, in a 64 QAMconstellation, each point is identified by a 6 bit grey code.

Because information and encoded bits are random, all 16 symbols in theconstellation shown in FIG. 1 are expected to occur with a relativelyequal probability given a sufficient observation length in theprocessing circuitry of the receiver 14. An optimal threshold shouldtherefore provide an equal likelihood for the bits b2 and b3. Themathematical expectation, or mean, of likelihood values for the bits b2and b3 should also be the same. If there is no noise in the receivedsignal, the threshold Sth_(i) will therefore fall in the middle of b2(or b3)=0 and b2 (or b3)=1 as shown in FIG. 2. If the received signal iscorrupted by noise, the distribution of the absolute values of the I andQ components will spread out and the threshold will be shifted to theright compared to the case in which no noise is present.

Taking this into account, an optimal value for the threshold Sth_(i) isgiven by the following formula:

${Sth}_{i} = {{\sqrt{E} \times \frac{2}{\sqrt{10}}} = {{A \times \frac{2}{\sqrt{10}}} = {A \times {const}}}}$where E and A are respectively estimates of the mean energy andamplitude of received symbols in the demodulator 19. The value of theconstant is given by

$\frac{2}{\sqrt{10}}$and is calculated, according to FIGS. 1 and 2, so that the thresholdSthi falls in the middle of b2 (or b3)=0 and b2 (or b3)=1 if there is nonoise. If the received signal also contains noise, then the estimatedata energy E also includes signal and noise variance and the estimatedthreshold is therefore larger than in the case of no noise. In thisinstance, the estimate can be used as an approximation of the optimalthreshold.

The above described example can be generalised to cover other square QAMconstellations with 4^(m) points. In this generalised case, the optimumvalue of the threshold Sth_(i)=A×2×i×Δ, where m is a positive integerand i is a positive integer from 1 to (√{square root over (4^(m-1))})−1.

It will be appreciated that the computation of A and the multiplicationof A by a constant to derive the optimal value of the threshold Sth_(i)is very simple and efficient to implement in hardware. One such hardwareimplementation is illustrated in FIG. 4, which shows a more detailedview of the log-likelihood estimator 20 of the receiver 14. In thisFigure, the channel estimates yI_(k) and yQ_(k) are received from thedemodulator 19. The average amplitude of the I channel estimate over ablock of K symbols is determined by a first averaging block 28.Similarly, the average amplitude of the Q channel estimates over a blockof K symbols is computed in the second averaging block 29.

The value of K is chosen so as to optimise receiver performance,depending upon the speed of change in channel condition. In fast fadingconditions, an excessively large value of K will prevent the calculatedthreshold from following the optimal threshold. However, a value of Kthat is too small will not provide enough statistical information for aproper threshold calculation to be performed. The value of K ispreferably inversely proportional to the speed of change in channelcondition.

The outputs of the averaging blocks 28 and 29 are input into aprocessing block 30 which approximates the mean amplitude A of eachreceived symbol according to the equation A≈max(AI, AQ)+0.5 min(AI, AQ),where AI and AQ are respectively the averages of orthogonal I and Qcomponents of each received symbol. The processing block 30 multipliesthat mean amplitude A by a constant, which in the case of a 16 QAMsignal is

$\frac{2}{\sqrt{10}}$to determine an optimised value of the threshold Sth_(i).

The calculated threshold value is then applied at the output of theprocessing block 30 to the inputs of subtraction blocks 31 and 32. Insubtraction block 31, the absolute value of the I channel estimate foreach symbol is subtracted from the threshold value Sth_(i) to derive thelog-likelihood of the bit b2. Similarly, the absolute magnitude of the Qchannel estimate for a received symbol is subtracted from the value ofthe threshold Sth_(i) to derive the value of the log-likelihood of thebit b3. The log-likelihoods of the bits b0 and b1 respectively equal thereceived I and Q channel estimates.

Accordingly, the log-likelihood estimator 20 provides 4 log-likelihoodoutputs L_(k)(b₀),L_(k)(b_(i)),L_(k)(b₂),L_(k)(b₃) corresponding to eachof 4 soft bits provided to the turbo decoder in use in demodulating the16 QAM signal. In this figure, the log-likelihood estimator 20approximates the amplitude A and the threshold Sth_(i) based on K inputsymbols, and then uses this approximated value for demodulating each ofthose K received symbols.

It will be appreciated that in other embodiments of the inventioninvolving square QAM constellations other than the 16 QAM constellationdescribed above, the same principle is used by the log-likelihoodestimator 20 the threshold Sth_(i) however the processing block 30 maybe required to calculate multiple threshold values Sth_(i)=A×2×i×Δ,where m>1 is a positive integer and i is a positive integer from 1 to(√{square root over (4^(m-1))})−1. For example, in the case of a 64 QAMsignal constellation, the thresholds Sth_(i(1/2/3)) will be equal to

$\frac{\left( {{2/4}/6} \right)}{\sqrt{42}}.$Similarly, for a 256 QAM signal constellation, the thresholdsSth_(i(1/2/3/4/5/6/7)) equals

$\frac{\left( {{{{{{2/4}/6}/8}/10}/12}/14} \right)}{\sqrt{170}}.$

By taking advantage of the randomness of data transmitted, thecomputation of the log-likelihood values is independent from the symbolprocessing, and can be realised as a stand alone block. Moreover, noassumptions about data to pilot ratio or proportional of pilot signalover total power need be made. The processing required to effect thiscomputation is very simple and efficient, and requires minimal powerconsumption and time.

Another hardware implementation of a log-likelihood estimator is shownin FIG. 5. In this example, the log-likelihood estimator 40 is identicalto the log-likelihood estimator 20 shown in FIG. 4, except for theprocessing block 41 which computes the threshold Sth_(i) in a differentmanner.

The processing block 41 approximates the mean amplitude A of eachreceived symbol according to the equation A=AI+AQ. The processing block41 then multiplies that mean amplitude A by a constant, which in thecase of a 16 QAM signal is 0.5 to determine an optimised value of thethreshold Sth_(i).

It will be appreciated that the hardware implementation shown in FIG. 5has the advantage of being simpler to realise that the hardwareimplementation shown in FIG. 1. The computation of the mean amplitude Aonly requires the addition of two quantities, namely AI and AQ. In thecase of a 16 QAM signal, the value of [AI+AQ] is slightly greater thanthe value of [max (AI, AQ)+0.5×min (AI, AQ)] computed by the processingblock 30 shown in FIG. 4. However, this is compensated by the fact thatthe factor 0.5, by which [AI, AQ] is multiplied, is slightly less thanfactor

$\frac{2}{\sqrt{10}}$(=0.632) by which [max (AI,AQ)+0.5×min (AI, AQ)] is multiplied.

The hardware implementations shown in FIGS. 4 and 5 are bothillustrative of a more general principle for computing a threshold. ??this used in demodulating a QAM signal to generate soft bits perreceived symbol for input to a turbo decoder, in which the meanamplitude A of the received symbols is computed, and the mean amplitudeA is multiplied by a constant C_(i) for a square QAM constellation with4^(m) points, such that Sth_(i)=A×C_(i), where m is a positive integerand i is a positive integer from 1 to (√{square root over (4^(m-1))})−1.FIGS. 4 and 5 illustrate two different techniques for computing the meanamplitude A.

The constant C_(i) in FIG. 4 is computed according to C_(i)2×i×Δ where Δis a normalising parameter for a square constellation with 4^(m) points,whilst the constant C_(i) in FIG. 5 is simply 0.5 for a 16 QAMconstellation.

In general, for demodulation of a QAM constellation with 4^(m) points,there will be (√{square root over (4^(m-1))})−1 different constantsC_(i) and correspondingly there will be (√{square root over(4^(m-1))})−1 different values of the threshold Sth_(i).

Finally, it is to be understood that various modifications and/oradditions may be made to the above described method or device withoutdeparting from the spirit or ambit of the invention.

1. A method on a communication receiver having a soft bit generator anda turbo decoder, the method for computing a threshold Sth_(i) used indemodulating a quadrature amplitude modulated (QAM) signal received bythe communication receiver to generate a plurality of soft bits perreceived symbol for input to the turbo decoder, the method including thesteps of: the soft bit generator: computing a mean amplitude A of thereceived symbols; multiplying the mean amplitude A by a constant C_(i)for a square QAM constellation with 4^(m) points, such thatSth _(i) =A×C _(i) where m is a positive integer and I is a positiveinteger from 1 to (√{square root over (4^(m-1))})−1; computing one ormore of the soft bits from the threshold Sth_(i); and outputting thecomputed soft bits to the turbo decoder.
 2. A method according to claim1, wherein the mean amplitude A is computed from a block of K receivedsymbols, where K is a positive integer.
 3. A method according to eitherone of claims 1 or 2, wherein the value of K is inversely proportionalto the speed of change in channel conditions.
 4. A method according toeither one of claims 1 or 2, wherein the constant C_(i) is computedaccording toC _(i)=2×I×Δ where Δ is a normalising parameter for a square QAMconstellation with 4^(m) points.
 5. A method according to claim 4,wherein the QAM signal is a 16QAM signal and the constant$C_{i}\mspace{11mu}\text{equals}\mspace{11mu}{\frac{2}{\sqrt{10}}.}$ 6.A method according to claim 4, wherein the QAM signal is a 16QAM signaland the constant Ci equals 0.5.
 7. A method according to claim 1,wherein the mean amplitude A of the received symbols is computedaccording toA=max(AI,AQ)+0.5min(AI,AQ) where AI and AQ are respectively the averagesof orthogonal I and Q components of each received symbol.
 8. A methodaccording to either one of claims 1 or 2, wherein the mean amplitude Aof the received symbols is computed according toA=AI+AQ where AI and AQ are respectively the averages of orthogonal Iand Q components of each received symbol.
 9. A method according to claim1, wherein log 2 4 m soft bits are computed from the threshold Sth_(i).10. A device within a communication receiver for computing a thresholdSth_(i) used in demodulating a quadrature amplitude modulated (QAM)signal received by the communication receiver to generate a plurality ofsoft bits per received symbol for input to a turbo decoder, the deviceincluding: means for computing a mean amplitude A of the receivedsymbols and multiplying the mean amplitude A by of the received symbolsand multiplying the mean amplitude A by a constant C_(i) for a squareQAM constellation with 4^(m) points, such thatSth _(i) =A×C _(i) where m is a positive integer and i is a positiveinteger from 1 to (√{square root over (4^(m-1))})−1; means for computingone or more of the soft bits from the threshold Sth_(i); and means foroutputting the computed soft bits to the turbo decoder.
 11. A devicewithin a communication receiver for generating soft bits per receivedsymbol for input to a turbo decoder used in demodulating a quadratureamplitude modulated (QAM) signal received by the communication receiver,the device including: means for computing a mean amplitude A of thereceived symbols and multiplying the mean amplitude A by a constantC_(i) for a square QAM constellation with 4^(m) points, such thatSth _(i) =A×C _(i) where m is a positive integer and i is a positiveinteger from 1 to (√{square root over (4^(m-1))})−1; and means forcomputing one or more of the soft bits from the threshold Sth_(i); andmeans for outputting the computed soft bits to the turbo decoder.